Medical device

ABSTRACT

A medical device is described having a handle, a shaft coupled to the handle, an end effector coupled to the shaft and an ultrasonic transducer coupled to the end effector for delivering ultrasonic energy to tissue or a vessel to be treated. A drive circuit is provided for generating a drive signal that is applied to the ultrasonic transducer. In one embodiment, the drive circuit includes a resonant circuit and a controller that varies the period of the drive signal passed through the resonant circuit in order to control the power supplied to the ultrasonic transducer.

The present invention relates to the field of medical devices and in particular, although not exclusively, to medical cauterization and cutting devices. The invention also relates to drive circuits and methods for driving such medical devices.

Many surgical procedures require cutting or ligating blood vessels or other internal tissue and many procedures are performed using minimally invasive techniques with a hand-held cauterization device to perform the cutting or ligating. Some existing hand-held cauterization devices use an ultrasonic transducer in the cauterization device to apply ultrasonic energy to the tissue to be cut or ligated. However, driving the ultrasonic transducer can be problematic due to inherent instabilities associated with the ultrasonic transducer that is used to generate the ultrasonic sound energy.

The present invention aims to provide an alterative circuit design that will allow the safe and reliable operation of such an ultrasonic cutting or cauterization device. The circuit design also advantageously allows for the miniaturisation of the circuitry so that it can be built into a hand-held cauterization device, whilst still being able to provide the power and control required for the medical procedure.

According to one aspect, the invention provides a medical device comprising: an input for receiving an input signal for controlling the medical device; an end effector; an ultrasonic transducer coupled to the end effector; a drive circuit coupled to the input and operable to generate a periodic drive signal and to provide the drive signal to the ultrasonic transducer; wherein the ultrasonic transducer has a resonant characteristic and wherein the drive circuit comprises a resonant circuit that is matched to the resonant characteristic of the ultrasonic transducer.

In one embodiment, a first controller may be provided for varying the period of the drive signal in dependence upon a sensed load voltage and a sensed load current applied to the transducer. The first controller may be arranged to vary the period of the drive signal in dependence upon a phase difference between the sensed load voltage and the sensed load current. This may be done so that the phase difference between the sensed load voltage and the sensed load current corresponds to a phase demand. A second controller may also be provided that is arranged to vary the phase demand in order to vary the power, voltage or current applied to the ultrasonic transducer. The second controller may vary the phase demand in dependence upon the sensed load voltage and/or the sensed load current or on a measured load impedance.

The device may also comprise switching circuitry that generates a periodic drive signal from a DC supply and wherein the resonant circuit is configured to receive the periodic drive signal.

In one embodiment, the device may comprise control circuitry that is arranged to generate a digital drive signal comprising periodic drive pulses of opposite polarity and wherein the relative timing of the pulses is selected to minimise third harmonic content of the drive signal.

In preferred embodiments, circuitry is provided for reducing harmonic components of the drive signal that are applied to the transducer. This circuitry may comprise one or more of: i) a snubber circuit connected in parallel with the transducer for providing a low impedance path for harmonic signals of the drive signal; and ii) an active feedback circuit that is arranged to generate harmonic compensation signals to cancel harmonic signals of the drive signal.

The device is preferably battery powered and comprises a battery compartment for holding one or more batteries for providing power to the drive circuit for generating said drive signal.

A control circuit may also be provided that receives a measurement of the drive signal and that varies the period of the drive signal to control the power, voltage and/or current delivered to the ultrasonic transducer. The measurement may be obtained from a sampling circuit that samples a sensed voltage or current signal at a sampling frequency that varies with the period and phase of the drive signal. The control circuit may be configured to vary the period of the drive signal so that the frequency of the drive signal varies around the resonant frequency of the ultrasonic transducer, preferably within 0.1% to 1% of the resonant frequency of the ultrasonic transducer. The resonant characteristic of the ultrasonic transducer may vary with a load coupled to the ultrasonic transducer during use and wherein the control circuit is configured to vary the period of the drive signal to track changes in the resonant characteristic of the ultrasonic transducer.

According to another aspect, the present invention also provides a medical device comprising: an input for receiving an input for controlling the operation of the device; an end effector; an ultrasonic transducer coupled to the end effector; a drive circuit operable to generate a periodic drive signal and to provide the drive signal to the ultrasonic transducer; and a control circuit operable to vary the period of the drive signal to control at least one of the power, the voltage and the current provided to the ultrasonic transducer.

The control circuit typically comprises a first controller for varying the period of the drive signal in dependence upon a sensed load voltage and a sensed load current applied to the ultrasonic transducer. The first controller may be arranged to vary the period of the drive signal in dependence upon a phase difference between the sensed load voltage and the sensed load current. The first controller may be arranged to vary the period of the drive signal so that the phase difference between the sensed load voltage and the sensed load current corresponds to a phase demand. The control circuit may include a second controller that is arranged to vary the phase demand in order to vary the power, voltage or current applied to the ultrasonic transducer. The second controller may vary the phase demand in dependence upon the sensed load voltage and/or the sensed load current or on a measured load impedance.

The present invention also provides a method of operating a medical device comprising generating a periodic drive signal and applying the drive signal to an ultrasonic transducer that is coupled to an end effector of the medical device and controlling the period of the drive signal to control at least one of the power, current, and voltage applied to the ultrasonic transducer.

According to another aspect, the invention provides a method of cauterising or cutting a vessel or tissue, the method comprising: gripping the vessel or tissue with an end effector of a medical device; using a drive circuit to apply a periodic drive signal to an ultrasonic transducer coupled to the end effector; and controlling the period of the drive signal to control at least one of the power, current, and voltage applied to the tissue to perform the cauterising or the cutting.

This method may use the above described medical device to perform the cauterising or cutting.

The controlling step may vary the period of the drive signal to control the power of the ultrasonic signal applied to the tissue or vessel, and the method may further comprise obtaining a measurement of the impedance of a load on the drive circuit caused by the transducer and the tissue or vessel and varying a desired power to be applied to the tissue or vessel in dependence upon the obtained impedance measurement.

According to another aspect, the present invention provides electronic apparatus for use in a medical device having an ultrasonic transducer, the electronic apparatus comprising: a drive circuit operable to generate a periodic drive signal for supply to the ultrasonic transducer; sensing circuitry for sensing a load voltage and/or a load current supplied to the ultrasonic transducer; a control circuit operable to vary the period of the drive signal to control at least one of the power, the voltage and the current supplied to the ultrasonic transducer in dependence upon the load voltage and/or load current sensed by the sensing circuitry.

The control circuit may be arranged to vary the period of the drive signal in dependence upon a phase difference between the sensed load voltage and the sensed load current. The control circuit may be arranged to vary the period of the drive signal so that the phase difference between the sensed load voltage and the sensed load current corresponds to a phase demand. A second control circuit may be provided that is arranged to vary the phase demand in order to vary the power, voltage or current applied to the ultrasonic transducer. The second control circuit may be arranged to vary the phase demand in dependence upon the sensed load voltage and/or the sensed load current.

Typically, the ultrasonic transducer has a resonant characteristic and the drive circuit comprises a resonant circuit that is matched with the resonant characteristic of the ultrasonic transducer; and the circuitry further comprising switching circuitry that generates a periodic drive signal from a DC supply and wherein the resonant circuit is configured to receive the periodic drive signal.

These and various other features and aspects of the invention will become apparent from the following detailed description of embodiments which are described with reference to the accompanying Figures in which:

FIG. 1 illustrates a hand-held cauterization device that has batteries and drive and control circuitry mounted in a handle portion of the device;

FIG. 2 is a part block part schematic diagram illustrating the main components of the cauterization device used in one embodiment of the invention;

FIG. 3 is a block diagram illustrating the main electrical components of the cauterization device shown in FIG. 2;

FIG. 4 is a timing diagram illustrating the form of a waveform generated by a bridge signal generator forming part of the electrical components shown in FIG. 3;

FIG. 5 is a block diagram that schematically illustrates processing modules that form part of the microprocessor shown in FIG. 2;

FIG. 6 is a plot illustrating the way in which the phase of a measured signal varies with the period of the waveform generated by the bridge signal generator and illustrating that the phase plot changes depending on a load gripped by the cauterization device;

FIG. 7 is a flow chart illustrating the processing performed by the microprocessor shown in FIG. 5;

FIG. 8 is a block diagram illustrating the form of snubber circuitry that can be added to the circuitry shown in FIG. 2; and

FIG. 9 is a block diagram illustrating the way in which harmonics of the drive signal can be actively cancelled using feedback circuitry and a controller within the microprocessor.

MEDICAL DEVICE

Many surgical procedures require cutting or ligating blood vessels or other vascular tissue. With minimally invasive surgery, surgeons perform surgical operations through a small incision in the patient's body. As a result of the limited space, surgeons often have difficulty controlling bleeding by clamping and/or tying-off transected blood vessels. By utilizing ultrasonic-surgical forceps, a surgeon can cauterize, coagulate/desiccate, and/or simply reduce bleeding by controlling the ultrasonic energy applied to the tissue/vessel by one or more ultrasonic transducers coupled to one or both of the jaw members of the surgical forceps.

FIG. 1 illustrates the form of an ultrasonic-surgical medical device 1 that is designed for minimally invasive medical procedures, according to one embodiment of the present invention. As shown, the device 1 is a self contained device, having an elongate shaft 3 that has a handle 5 connected to the proximal end of the shaft 3 and an end effector 7 connected to the distal end of the shaft 3. In this embodiment, the end effector 7 comprises medical forceps 9 that are controlled by the user manipulating control levers 11 and 13 of the handle 5.

During a surgical procedure, the shaft 3 is inserted through a trocar to gain access to the patient's interior and the operating site. The surgeon will manipulate the forceps 9 using the handle 5 and the control levers 11 and 13 until the forceps 9 are located around the vessel to be cut or cauterised. Electrical energy is then applied, in a controlled manner, to an ultrasonic transducer 8 that is mounted within the handle 5 and coupled to the forceps 9 via a waveguide (not shown) within the shaft 3, in order to perform the desired cutting/cauterisation using ultrasonic energy. As shown in FIG. 1, in this embodiment, the handle 5 also houses batteries 15 and control electronics 17 for generating and controlling the electrical energy required to perform the cauterisation. In this way, the device 1 is self contained in the sense that it does not need a separate control box and supply wire to provide the electrical energy to the forceps 9. However, such a separate control box may be provided if desired.

SYSTEM CIRCUITRY

FIG. 2 is a schematic block diagram illustrating the main electrical circuitry of the cauterization/cutting device 1 used in this embodiment to generate and control the electrical energy supplied to the ultrasonic transducer 8. As will be explained in more detail below, in this embodiment, the circuitry is designed to control the period of an electrical drive waveform that is generated and applied to the ultrasonic transducer 8 in order to control the amount of power supplied thereto. The way that this is achieved will become apparent from the following description.

As shown in FIG. 2, the cauterization/cutting device 1 comprises a user interface 21—via which the user is provided with information (such as indicating that energy is being applied to the ultrasonic transducer 8) and through which the user controls the operation of the cauterization/cutting device 1. As shown, the user interface 21 is coupled to a microprocessor 23 that controls the cauterisation/cutting procedure by generating control signals that it outputs to gate drive circuitry 25. In response to the control signals from the microprocessor, the gate drive circuitry 25 generates gate control signals that cause a bridge signal generator 27 to generate a desired drive waveform that is applied to the ultrasonic transducer 8 via a drive circuit 29. Voltage sensing circuitry 31 and current sensing circuitry 33 generate measures of the current and voltage applied to the ultrasonic transducer 8, which they feed back to the microprocessor 23 for control purposes. FIG. 2 also shows the batteries 15 that provide the power for powering the electrical circuitry shown in FIG. 2. In this embodiment, the batteries 15 are arranged to supply 0V and 14V rails.

FIG. 3 illustrates in more detail the components of the gate drive circuitry 25, the bridge signal generator 27, the drive circuit 29, the voltage sensing circuitry 31 and the current sensing circuitry 33 that are used in this embodiment. FIG. 3 also shows an electrical equivalent circuit 35 of the piezo-electric ultrasonic transducer 8 and the load (R_(load)) formed by the tissue/vessel to be treated.

As shown in FIG. 3, the gate drive circuitry 25 includes two FET gate drives 37—FET gate drive 37-1 and FET gate drive 37-2. A first set of control signals (CTRL₁) from the microprocessor 23 is supplied to FET gate drive 37-1 and a second set of control signals (CTRL₂) from the microprocessor 23 is supplied to FET gate drive 37-2. FET gate drive 37-1 generates two drive signals—one for driving each of the two FETs 41-1 and 41-2 of the bridge signal generator 27. In response to the signals from the microprocessor 23, the FET gate drive 37-1 generates drive signals that causes the upper FET (41-1) to be on when the lower FET (41-2) is off and vice versa. This causes the node A to be alternately connected to the 14V rail (when FET 41-1 is switched on) and the 0V rail (when the FET 41-2 is switched on). Similarly, in response to the signals from the microprocessor 23, the FET gate drive 37-2 generates two drive signals—one for driving each of the two FETs 41-3 and 41-4 of the bridge signal generator 27. The FET gate drive 37-2 generates drive signals that causes the upper FET (41-3) to be on when the lower FET (41-4) is off and vice versa. This causes the node B to be alternately connected to the 14V rail (when FET 41-3 is switched on) and the 0V rail (when the FET 41-4 is switched on). Thus the two sets of control signals (CTRL₁ and CTRL₂) output by the microprocessor 23 control the digital waveform that is generated and applied between nodes A and B. Each set of control signals (CTRL₁ and CTRL₂) comprises of a pair of signal lines, one to indicate when the high side FET is on and the other to indicate when the low side FET is on. Thus the microprocessor 23, either through software or through a dedicated hardware function, can ensure that the undesirable condition when both high and low side FETs are simultaneously turned on does not occur. In practice this requires leaving a dead time when both high and low side FETs are turned off to ensure that, even when allowing for variable switching delays, there is no possibility that both FETs can be simultaneously on. In the present embodiment a dead time of about 100 ns was used.

As shown in FIG. 3, the nodes A and B are connected to the drive circuitry 29, thus the digital voltage generated by the bridge signal generator 27 is applied to the drive circuit 29. This applied voltage will cause a current to flow in the drive circuit 29. The digital waveform of the voltage that is applied between nodes A and B (V_(AB)) is designed to have a fundamental frequency (f_(d)) and very little third harmonic component (3f_(d)); and is illustrated in FIG. 4. As shown, the voltage V_(AB) has a first pulse 39-1 and a second pulse 39-2. The first pulse is generated by the FETs 41-1 and 41-2 in dependence upon the first set of control signals (CTRL₁) from the microprocessor 23. The second pulse is generated by the FETs 41-3 and 41-4 in dependence upon the second set of control signals (CTRL₂) from the microprocessor 23. The pulses are periodically produced, with a period Δt. The pulses 39 are spaced apart from each other and from the beginning and end of the period by time periods that are selected to minimise the 3^(rd) harmonic content of the waveform. The inventors have found that minimising the 3^(rd) harmonic component is important for the stable driving of the piezo-electric ultrasonic transducer 8.

As shown in FIG. 3, the drive circuit 29 includes a capacitor-inductor-inductor resonant circuit 43 formed by capacitor C_(s) 45, inductor L_(s) 47 and inductor L_(m) 49. The microprocessor 23 is arranged to generate control signals for the gate drive circuitry 25 so that the fundamental frequency (f_(d)=1/Δt) of the digital voltage applied across nodes A and B is around the resonant frequency of the resonant circuit 43, which in this embodiment is about 50 kHz. As a result of the resonant characteristic of the resonant circuit 43, and particularly in conjunction with the resonant characteristic of the piezo-electric ultrasonic transducer 35, the digital voltage applied across nodes A and B will cause a substantially sinusoidal current at the fundamental frequency (f_(d)) to flow within the resonant circuit 43. This is because the drive voltage (V_(AB)) has minimal 3 ^(rd) harmonic content and the higher harmonic content will be significantly attenuated by the resonant circuit 43.

As illustrated in FIG. 3, the inductor L_(m) 49 is the primary of a transformer 51, the secondary of which is formed by inductor L_(sec) 53. The transformer 51 up-converts the drive voltage (V_(d)) across inductor L_(m) 49 to the load voltage (V_(L); typically about 120 volts) that is applied to the ultrasonic transducer 8. The electrical characteristics of the ultrasonic transducer 8 change with the impedance of the forceps' jaws and any tissue or vessel gripped by the forceps 9; and FIG. 3 models the ultrasonic transducer 8 and the impedance of the forceps' jaws and any tissue or vessel gripped by the forceps 9 by the inductor L_(t) 57, the parallel capacitors C_(t1) 59 and C_(t2) 61 and the resistance R_(load). The inductor L_(s) and capacitor C_(s) of the drive circuit 29 are designed to have a matching LC product to that of inductor L_(t) and capacitor C_(t1) of the ultrasonic transducer 8. Matching the LC product of a series LC network ensures that the resonant frequency of the network is maintained. Similarly, the magnetic reactance of the inductor, Lsec, is chosen so that at resonance it matches with the capacitive reactance of the capacitor C_(t2) of the ultrasonic transducer 8. For example, if the transducer 8 is defined such that capacitor C_(t2) has a capacitance of about 3.3 nF, then the inductor Lsec should have an inductance of about 3 mH (at a resonant frequency of about 50 kHz). Designing the drive circuit 29 in this way provides for the optimum drive efficiency in terms of energy delivery to the tissue/vessel gripped by the forceps 9. The efficiency improvement is realised because the current flowing in C_(s) and consequently the FET bridge 27 is reduced, because the transformer magnetising current cancels out the current flowing in C_(t2.) In addition, because of this current cancellation, the current flowing in C_(s) is proportional to the current flowing in Rload, which allows the load current to be determined by measuring the current flowing in C_(s).

However, it is not always desired to apply full power to tissue/vessel to be treated. Therefore, in this embodiment, the amount of ultrasonic energy supplied to the vessel/tissue is controlled by varying the period (At) of the digital waveform applied across nodes A and B so that the drive frequency (f_(d)) moves away from the resonant frequency of the drive circuit/ultrasonic transducer 8. This works because the ultrasonic transducer 8 acts as a frequency dependent (lossless) attenuator. The closer the drive signal is to the resonant frequency of the ultrasonic transducer 8, the more ultrasonic energy the ultrasonic transducer 8 will transfer to the tissue. Similarly, as the frequency of the drive signal is moved away from the resonant frequency of the ultrasonic transducer 8, less and less ultrasonic energy is transferred to the tissue by the ultrasonic transducer 8. In addition or instead, the duration of each of the pulses 39 may be varied to control the amount of ultrasonic energy delivered to the tissue/vessel.

In this embodiment, the microprocessor 23 controls the power delivery based on a desired power to be delivered to the circuitry 35 (which models the ultrasonic transducer 8 and the tissue/vessel gripped by the forceps 9) and measurements of the load voltage (V_(L)) and of the load current (i_(L)) obtained from the voltage sensing circuitry 31 and the current sensing circuitry 33. As shown in FIG. 3, in this embodiment, the current sensing circuitry 33 is formed by placing an additional inductor turn(s) 67 adjacent inductor 47 (or inductor 49) and sensing the voltage obtained from the coupled inductor 67. This produces a voltage that is related to the current flowing through the primary side of the transformer by V=Ldi/dt; and which can be converted into a suitable measure of the load current (i_(L)) for example by integration and scaling to account for the inductance of the inductor 67 and to take into account the ratio of the number of turns between inductor 47 and inductor 67 and the ratio of the number of turns between inductor 49 and inductor 53. One of the advantages of using this method of obtaining a measure of the current through the load is that the measurement is not sensitive to the current circulating through L_(sec) and C_(t2) on the secondary side of the transformer circuit. This is advantageous as this circulating current can be quite large compared to the current through the load (R_(load)), but does not contribute to ultrasonic energy delivery to the tissue. As shown, the signal obtained from the current sensor 33 is passed to the microprocessor 23.

FIG. 3 also shows that the voltage sensing circuitry 31 senses the load voltage (V_(L)) via a resistive divider circuit (although a capacitive divider circuit could be used instead). The sensed voltage obtained from the voltage sensing circuitry 31 is passed to the microprocessor 23. Although not shown in FIG. 3, the output from the current sensing circuitry 33 and the output from the voltage sensing circuitry 31 would typically be passed through an op-amp circuit before being input to the microprocessor 23, to provide a DC bias and reduce the signal levels to values suitable for input to the microprocessor 23.

MICROPROCESSOR

FIG. 5 is a block diagram illustrating the main components of the microprocessor 23 that is used in this embodiment. As shown, the microprocessor 23 includes synchronous I,Q sampling circuitry 81 that receives the sensed voltage and current signals from the sensing circuitry 31 and 33 and obtains corresponding samples which are passed to a measured voltage and current processing module 83. The measured voltage and current processing module 83 uses the received samples to calculate the impedance of, the RMS voltage applied to and the current flowing through, the ultrasonic transducer 8 and the tissue/vessel gripped by the forceps 9; and from them the power that is presently being supplied to the ultrasonic transducer 8 and the tissue/vessel gripped by the forceps 9. The determined values are then passed to a power controller 85 for further processing. The measured voltage and current processing module 83 also processes the received I and Q samples to calculate the phase difference between the load voltage (V_(L)) and the load current (i_(L)). At resonance, this phase difference should be around zero. The measured phase difference is also passed to the power controller 85 and also to a phase lock loop (PLL) controller 87.

The power controller 85 uses the received impedance value and the delivered power value to determine, in accordance with a predefined algorithm and a power set point value received from a medical device control module 89, a set point phase value (Phase Demand), which is passed to the PLL controller 87. The medical device control module 89 is in turn controlled by signals received from a user input module 91 that receives inputs from the user (for example pressing buttons or activating the control levers 11 or 13 on the handle 5) and also controls output devices (lights, a display, speaker or the like) on the handle 5 via a user output module 93. The PLL controller 87 uses the received Phase Demand and the latest measured phase difference and determines a new waveform period (Δt_(new)) that it outputs to the control signal generator 95 to try to force the measured phase towards the Phase Demand. The control signal generator 95 changes the control signals CTRL₁ and CTRL₂ in order to change the waveform period to match the new period Δt_(new). As those skilled in the art will appreciate, both the CTRL control signals will comprise periodic pulses with the period corresponding to Δt_(new). The relative timing of the pulses of the two control signals is set to minimise the 3^(rd) order harmonic of the waveform that is generated by the bridge signal generator 27. In this embodiment, the control signal CTRL₁ is output to the FET gate drive 37-1 (shown in FIG. 2), which amplifies the control signal and then applies it to the FET 41-1. The FET gate drive 37-1 also inverts the amplified control signal and then applies it to the FET 41-2. Similarly, the control signal CTRL₂ is output to the FET gate drive 37-2 (shown in FIG. 2), which amplifies the control signal and then applies it to the FET 41-3. The FET gate drive 37-2 also inverts the amplified control signal and then applies it to the FET 41-4, thereby generating the desired waveform with the new period (Δt_(new)).

I & Q SIGNAL SAMPLING

Both the load voltage and the load current will be substantially sinusoidal waveforms, although they may be out of phase, depending on the impedance of the load represented by the transducer 8 and the vessel/tissue gripped by the forceps 9. The load current and the load voltage will be at the same drive frequency (f_(d)) corresponding to the presently defined waveform period (Δt_(new)). Normally, when sampling a signal, the sampling circuitry operates asynchronously with respect to the frequency of the signal that is being sampled. However, as the microprocessor 23 knows the frequency and phase of the switching signals, the synchronous sampling circuit 81 can sample the measured voltage/current signal at predefined points in time during the drive period. In this embodiment, the synchronous sampling circuit 81 oversamples the measured signal eight times per period to obtain four I samples and four Q samples. Oversampling allows for a reduction of errors caused by harmonic distortion and therefore allows for the more accurate determination of the measured current and voltage values. However, oversampling is not essential and indeed under sampling (sampling once per period or less) is possible due to the synchronous nature of the sampling operation. The timing at which the synchronous sampling circuit 81 obtains these samples is controlled, in this embodiment, by the control signals CTRL₁ and CTRL_(2.) Thus when the period of these control signals is changed, the period of the sampling by the synchronous sampling circuit 81 also changes (whilst their relative phases stay the same). In this way, the sampling circuitry 81 continuously changes the timing at which it samples the sensed voltage and current signals as the period of the drive waveform is changed so that the samples are always taken at the same time points within the period of the drive waveform. Therefore, the sampling circuit 81 is performing a “synchronous” sampling operation instead of a more conventional sampling operation that just samples the input signal at a fixed sampling rate defined by a fixed sampling clock. Of course, such a conventional sampling operation could instead be used.

MEASUREMENTS

The samples obtained by the synchronous sampling circuitry 51 are passed to the measured voltage and current processing module 83 which can determine the magnitude and phase of the measured signal from just one “I” sample and one “Q” sample of the load current and load voltage. However, in this embodiment, to achieve some averaging, the processing module 83 averages consecutive samples to provide average “I” and “Q” values; and then uses the average I and Q values to determine the magnitude and phase of the measured signal. As discussed above, eight samples per period are obtained in this embodiment and these samples are used to compute the in-phase (I) and quadrature phase (Q) components of both the voltage and current according to the following formula.

$V_{I} = {\frac{1}{4}{\sum\limits_{k = 0}^{7}\; {{\sin \left( {k\; {\pi/4}} \right)}v_{k}}}}$ $V_{Q} = {\frac{1}{4}{\sum\limits_{k = 0}^{7}\; {{\cos \left( {k\; {\pi/4}} \right)}v_{k}}}}$ $I_{I} = {\frac{1}{4}{\sum\limits_{k = 0}^{7}\; {{\sin \left( {k\; {\pi/4}} \right)}i_{k}}}}$ $I_{Q} = {\frac{1}{4}{\sum\limits_{k = 0}^{7}\; {{\cos \left( {k\; {\pi/4}} \right)}i_{k}}}}$ ${{Power}({total})} = {\frac{1}{8}{\sum\limits_{k = 0}^{7}\; {v_{k}i_{k}}}}$

Where v_(k) and i_(k) represent the kth voltage and current sample respectively; and Power (total) is the total power delivered to the load including harmonic content. For the purposes of computational efficiency, the sine and cosine results can be pre-computed and stored in a look up table.

Of course, it should be recognised that some pre-processing of the data may be required to convert the actual measured I and Q samples into I and Q samples of the load voltage or the load current, for example, scaling, integration or differentiation of the sample values may be performed to convert the sampled values into true samples of the load voltage (V_(L)) and the load current (i_(L)). Where integration or differentiation is required, this can be achieved simply by swapping the order of the I and Q samples—as integrating/differentiating a sinusoidal signal simply involves a 90 degree phase shift.

The RMS load voltage, the RMS load current and the delivered power (P_(delivered)) can then be determined from:

$V_{RMS} = {\frac{1}{\sqrt{2}}\sqrt{\left( {V_{I}^{2} - V_{Q}^{2}} \right)}}$ $I_{RMS} = {\frac{1}{\sqrt{2}}\sqrt{\left( {I_{I}^{2} - I_{Q}^{2}} \right)}}$ $\begin{matrix} {{Power} = {V \cdot I^{*}}} \\ {= {\frac{1}{\sqrt{2}}\left( {V_{I} + {j\; V_{Q}}} \right)\left( {I_{I} - {j\; I_{Q}}} \right)}} \\ {= {P_{delivered} + {j\; P_{reactive}}}} \end{matrix}$ $P_{delivered} = {\frac{1}{\sqrt{2}}\left( {{V_{I}V_{I}} + {V_{Q}I_{Q}}} \right)}$ $P_{reactive} = {\frac{1}{\sqrt{2}}\left( {{V_{Q}I_{I}} - {V_{I}I_{Q}}} \right)}$ Power = V_(RMS)I_(RMS) = P_(delivered) + j P_(reactive)

In general, it is not necessary to compute the RMS voltage and current (which would require the computation of a square root), instead much of the control functions operate using V_(RMS) ² and I_(RMS) ². Delivered power can also be calculated directly from the individual samples, shown above. (Note that apparent power does not equal delivered power unless the impedance is purely real.)

The impedance of the load represented by the ultrasonic transducer 8 and the vessel/tissue gripped by the forceps 9 can be determined from:

$\begin{matrix} {Z_{Load} = \frac{\left( {V_{I} + {j\; V_{Q}}} \right)}{\left( {I_{I} + {j\; I_{Q}}} \right)}} \\ {= \frac{\left( {V_{I} + {j\; V_{Q}}} \right)\left( {I_{I} - {j\; I_{Q}}} \right)}{\left( {I_{I} + {j\; I_{Q}}} \right)\left( {I_{I} - {j\; I_{Q}}} \right)}} \\ {= \frac{\left( {{V_{I}I_{I}} + {V_{Q}I_{Q}} + {j\; V_{Q}I_{I}} - {j\; V_{I}I_{Q}}} \right)}{\sqrt{2}I_{RMS}^{2}}} \\ {= {R_{Load} + {j\; X_{Load}}}} \end{matrix}$

An alternative way of computing R_(Load) and X_(Load) is as follows:

$X_{Load} = \frac{P_{reactive}}{\sqrt{2}I_{RMS}^{2}}$

and the phase difference between the load voltage and the load current can be determined from:

Phase_(measured)=α tan 2(P _(reactive) , P _(delivered))

A computationally efficient, approximation to the a tan 2 function can be made using look up tables and interpolation in fixed point arithmetic, or using a ‘CORDIC’ like algorithm.

LIMITS

As with any system, there are certain limits that can be placed on the power, current and voltage that can be delivered to the ultrasonic transducer 8. The limits used in this embodiment and how they are controlled will now be described.

In this embodiment, the drive circuitry 29 is designed to deliver ultrasonic energy into tissue with the following requirements:

1) Supplied with a nominally 14V DC supply

2) Substantially sinusoidal output waveform at approximately 50 kHz

3) Power limited output of 90 W

4) Current limited to 1.4 A_(rms) and voltage limited to 130V_(rms)

5) Measured phase greater than a system defined phase limit

The power controller 85 maintains data defining these limits and uses them to control the decision about whether to increase or decrease the Phase Demand given the latest measured power, load impedance and measured phase. In this embodiment, the phase limit that is used depends on the measured load impedance. In particular, the power controller 85 maintains a look up table (not shown) relating load impedance to the phase limit; and the values in this table limit the phase so that when the measured load impedance is low (indicating that the jaws of the forceps 9 are open and not gripping tissue or a vessel), the delivered power is reduced (preferably to zero).

PHASE CHARACTERISTIC AND PHASE CONTROL

As mentioned above, the amount of ultrasonic energy supplied to the forceps 9 is controlled by varying the period (Δt) of the drive waveform (V_(AB)). This is achieved by utilising the fact that the impedance of the ultrasonic transducer 8 changes rapidly with the period (Δt) of the drive waveform. Therefore, by changing the period of the drive waveform (V_(AB)), the magnitude of the current through the ultrasonic transducer 8 changes and this can be used to regulate the ultrasonic energy delivered to the load. Maximum ultrasonic energy delivery will be achieved when the period of the drive waveform (V_(AB)) corresponds to the reciprocal of the resonant frequency of the ultrasonic transducer 8. Further, as the resonant frequency of the drive circuit 29 is designed to be matched with the resonant frequency of the ultrasonic transducer 8, when operating at this period, the measured phase will be approximately zero.

However, as those skilled in the art will appreciate, the resonant circuit 43 and the ultrasonic transducer 8 are coupled to a load whose impedance will vary during the surgical procedure. Indeed the medical device control module 89 uses this variation to determine whether the tissue or vessel has been cauterised, coagulated/desiccated. The varying impedance of the load changes the frequency characteristic of the ultrasonic transducer 8 and hence the current that flows through the resonant circuit 43. This is illustrated in FIG. 6, which is a phase plot 101 illustrating the way in which the measured phase difference between the load current and the load voltage varies with the period (frequency) of the drive waveform (V_(AB)) for a fixed value of load impedance. As the impedance of the load increases or decreases, the phase plot 101 will change shape, with the net effect being that the zero degree phase point will move up or down the period axis (left and right as represented by the arrows 103). Therefore, the PLL controller 87 must operate quickly enough to track the changes in the phase plot 101. In this embodiment, phase measurements are updated once every few cycles of the drive signal, so the phase measurement update rate is approximately 100 μs, which is fast enough to track most changes in the phase plot 101.

However, sudden changes in the phase plot 101 can occur that the PLL controller 87 cannot track. This can cause problems because, as illustrated in FIG. 6, the phase plot 101 has a minimum just after the zero crossing where the phase begins to rise again with reducing period (Δt). In this embodiment, the microprocessor 23 maintains a limit on the measured phase (phase_(limit)) which is greater than the minimum shown in FIG. 6 and which ensures that the phase is always kept above this hard limit. In this way, the microprocessor 23 can ensure that the phase does not go past the minimum point which can cause unstable operation of the ultrasonic transducer 8. If the measured phase reaches the phase limit, then the power controller 85 resets the Phase Demand to a value where the PLL controller 87 will keep the measured phase away from the minimum value. For example, the power controller 85 may change the Phase Demand to 40 degrees where there is a known and stable response to the subsequent variation of the Phase Demand to control the applied power. Once, reset in this way, the power controller 85 can then start to reduce the Phase Demand again so that the PLL controller 87 can then decrease the waveform period (Δt) in order to reduce the phase difference towards zero degrees where maximum power delivery will be applied. Of course, if the power controller 85 has set a Phase Demand signal to a value above zero then the PLL controller 87 will vary the waveform period until the measured phase matches the Phase Demand.

FIG. 7 is a flow chart illustrating the processing performed in this embodiment by the microprocessor 23. As shown, at the beginning of the process in step s1, the microprocessor 23 turns on the drive signal at a system defined maximum waveform period (Δt_(max)) by setting an initial Phase Demand from the power controller 85 and an initial waveform period output from the PLL controller 87. Provided the microprocessor 23 has not received, in step s3, a power down signal, the processing proceeds to step s5 where the measured voltage and current processing module 83 obtains the voltage and current samples from the synchronous sampling circuitry 81. In step s6 the measured voltage and current processing module 83 calculates the RMS load voltage, the RMS load current, the delivered power, the load impedance and the phase difference between the load voltage and the load current. The measured values are passed to the power controller 85 and the measured phase is also passed to the PLL controller 87. The power controller 85 compares, in step s7, the received phase measure with the defined phase limit and provided the measured phase is greater than the phase limit, the processing proceeds to step s9 where the power controller 85 compares the received voltage, current and power values with the defined limits for the applied voltage, current and power. The voltage and current limits are static limits that are defined in advance. However, as discussed above, the phase limit depends on the measured load impedance; and the power limit depends on the medical procedure and is defined by the power set point (P_(set)) provided by the medical device control module 89. If the power controller 85 determines at step s7 that the measured phase is less than (or equal to) the present phase limit then the power controller 85 resets the Phase Demand, in step s10, to a stable value as discussed above and the processing returns to step s3. If each of the measured values is below the corresponding limit then, in step s11, the power controller 85 decreases the Phase Demand that is passed to the PLL controller 87. At the start of the processing, the waveform period (Δt) is set to a defined maximum value (in this embodiment 20.2 μsec) which, with the circuitry used in this embodiment, should correspond to a phase difference between the load voltage and the load current of approximately 90 degrees. Therefore, regardless of the load, the initial waveform period should be on the left hand side of the zero crossing point on the phase plot 101 shown in FIG. 6. By decreasing the Phase Demand, the waveform period (Δt) will get closer to the zero crossing point on the phase plot 101, corresponding to the resonant frequency of the ultrasonic transducer 8. As a result, the applied current will increase and more ultrasonic energy will be delivered to the load. The processing then returns to step s3 and the above process is iteratively repeated.

Therefore, the current and power applied to the load should increase until one of the limits is reached. If the power controller 85 determines, in step s9, that a voltage, current or power limit has been reached, then the processing proceeds to step s13, where the power controller 85 increases the Phase Demand sent to the PLL controller 87 which will increase the waveform period (Δt) accordingly. This will cause the waveform period to move away from the resonant frequency of the ultrasonic transducer 8 and so the current and power delivered to the load will reduce. The processing then returns to step s3 as before.

Thus, by starting on the left hand side of the zero crossing point and slowly moving the waveform period (Δt) towards and away from the zero crossing point in the phase plot 101, the current and power level applied to the load can be controlled within the defined limits even as the impedance of the load changes and the resonant characteristic of the ultrasonic transducer 8 changes as the tissue/vessel is cut/cauterised.

MEDICAL DEVICE CONTROL MODULE

As mentioned above, the medical device control module 89 controls the general operation of the cutting/cauterisation device 1. It receives user inputs via the user input module 91. These inputs may specify that the jaws of the forceps 9 are now gripping a vessel or tissue and that the user wishes to begin cutting/cauterisation. In response, in this embodiment, the medical device control module 89 initiates a cutting/cauterisation control procedure. Initially, the medical device control module 89 sends an initiation signal to the power controller 85 and obtains the load impedance measurements determined by the measured voltage and current processing module 83. The medical device control module 89 then checks the obtained load impedance to make sure that the load is not open circuit or short circuit. If it is not, then the medical device control module 89 starts to vary the power set point to perform the desired cutting/cauterisation.

MODIFICATIONS AND ALTERNATIVES

A medical cauterisation device has been described above. As those skilled in the art will appreciate, various modifications can be made and some of these will now be described. Other modifications will be apparent to those skilled in the art.

In the above embodiment, the drive voltage generated by the bridge signal generator was designed to have minimal 3^(rd) order harmonic content. In addition or instead of using such a design of the drive voltage waveform, a snubber circuit may be provided to snub out or attenuate the 3^(rd) harmonic so that it is not applied to the ultrasonic transducer 8. FIG. 8 is a circuit diagram illustrating the way in which such a snubbing circuit 121 may be added to the circuitry shown in FIG. 3. As shown, the snubbing circuit 121 comprises an inductor-capacitor-resistor combination that is connected in series across the terminals of the transducer 8. The values of the inductor L_(snub) and the capacitor C_(snub) are chosen so the snubber circuit 121 has minimal impedance at the 3^(rd) harmonic (3f_(c)) of the resonant frequency of the resonant circuit 43. As a result, the snubber circuit 121 effectively provides a low impedance path for the 3^(rd) order harmonic of the drive signal, which thereby reduces the 3^(rd) order harmonics that are applied to the ultrasonic transducer 8. As those skilled in the art will appreciate, it is not essential that the snubber circuit 121 comprises an inductor-capacitor-resistor combination, a similar snubbing function could be achieved using just a capacitor-resistor combination.

In addition or alternatively, an active harmonic cancellation circuit may be provided that dynamically adds harmonic signals to cancel out the corresponding harmonics in the drive signal. The way in which such an active harmonic cancellation circuit could be provided is illustrated in FIG. 9. As shown, the voltage obtained from the voltage sensing circuit 31 is fed back to the microprocessor 23 and will include the desired fundamental frequency of the drive signal and any unwanted higher frequency harmonics (predominately the 3^(rd) harmonic if the drive waveform has not been designed to minimise 3^(rd) order harmonics) of the drive signal (V_(AB)) applied at terminals A and B as before. In this embodiment, the drive signal may just be a periodic square wave signal having the presently specified period (Δt) rather than the more complicated waveform illustrated in FIG. 4. As shown in FIG. 9, this feedback signal is passed to a negative input of combiner 133 where the feedback signal is subtracted from a reference signal (V_(REF)) that has been passed through a delay filter 131 that is used to delay the reference signal for stability of the feedback loop. The reference signal (V_(REF)) is generated by the microprocessor 23 (for example by the control signal generator 95) and typically it is a sinusoidal signal having the same period (Δt) and the same phase as presently set for the drive signal (V_(AB)). The error signal (e) output from the combiner 133 is input to a controller 135 (such as a PID controller) which is arranged to generate and output control signals which are amplified by an amplifier 137 (preferably a linear amplifier) and then applied to an inductor coil 139 (L_(H)) that is mutually coupled to the secondary inductor coil 53. Therefore, the signal applied to the ultrasonic transducer 8 includes the drive signal and the harmonic compensating signal from the inductor 139. Thus, by virtue of the controller 135 outputting control signals that seek to reduce the error signal (e) to zero, the content of the control signals will track and substantially cancel out the harmonic content of the drive signal, so that only the fundamental component having period (Δt) will be applied to the ultrasonic transducer 8.

In the above embodiment, various operating frequencies, currents, voltages etc were described. As those skilled in the art will appreciate, the exact currents, voltages, frequencies, capacitor values, inductor values etc. can all be varied depending on the application and any values described above should not be considered as limiting in any way. However, in general terms, the circuit described above has been designed to provide a drive signal to a medical device, where the delivered power is desired to be at least 10 W and preferably between 10 W and 200 W; the delivered voltage is desired to be at least 20 V_(RMS) and preferably between 30 V_(RMS) and 120 V_(RMS); the delivered current is designed to be at least 0.5 A_(RMS) and preferably between 1 A_(RMS) and 2 A_(RMS); and the drive frequency is desired to be at least 20 kHz and preferably between 30 kHz and 80 kHz.

In the above embodiment, the resonant circuit 43 was formed from capacitor-inductor-inductor elements. As those skilled in the art will appreciate, other resonant circuit designs with multiple capacitors and inductors in various series and parallel configurations or simpler LC resonant circuits may also be used. Also, in some applications there is no need for a transformer to step-up the drive voltage, as the FETs can deliver the required drive voltage.

FIG. 1 illustrates one way in which the batteries and the control electronics can be mounted within the handle of the medical device. As those skilled in the art will appreciate, the form factor of the handle may take many different designs. Indeed, it is not essential for the device to be battery powered, although this is preferred for some applications to avoid the need for power cords and the like.

In the above embodiment, an exemplary control algorithm for performing the cutting/cauterisation of the vessel or tissue gripped by the forceps was described. As those skilled in the art will appreciate, various different procedures may be used and the reader is referred to the literature describing the operation of such cutting/cauterisation devices for further details.

In the above embodiments, four FET switches were used to convert the DC voltage provided by the batteries into an alternating signal at the desired frequency. As those skilled in the art will appreciate, it is not necessary to use four switches—two switches may be used instead (using a half bridge circuit). Additionally, although FET switches were used, other switching devices, such as bipolar transistor switches may be used instead. However, MOSFETs are preferred due to their superior performance in terms of low losses when operating at the above described frequencies and current levels.

In the above embodiment, the I & Q sampling circuitry 81 sampled the sensed voltage/current signal eight times every period. As those skilled in the art will appreciate, this is not essential. Because of the synchronous nature of the sampling, samples may be taken more than once per period or once every n^(th) period if desired. The sampling rate used in the above embodiment was chosen to maximise the rate at which measurements were made available to the power controller 85, the PLL controller 87 and the medical device control module 89 as this allows for better control of the applied power during the cutting/cauterisation process.

In the above embodiment, a 14V DC supply was provided. In other embodiments, lower (or higher) DC voltage sources may be provided. In this case, a larger (or smaller) transformer turns ratio may be provided to increase the load voltage to the desired level or lower operating voltages may be used.

In the above embodiment, the medical device was arranged to deliver a desired power to the ultrasonic transducer. In an alternative embodiment, the device may be arranged to deliver a desired current or voltage level to the ultrasonic transducer.

In the above embodiment the battery is shown integral to the medical device. In an alternative embodiment the battery may be packaged so as to clip on a belt on the surgeon or simply be placed on the Mayo stand. In this embodiment a relatively small two conductor cable would connect the battery pack to the medical device.

In the above embodiment, a microprocessor based control circuitry was provided. This is preferred due to the ease with which the microprocessor can be programmed to perform the above control actions using appropriate computer software. Such software can be provided on a tangible carrier, such as a CD-ROM or the like. Alternatively, hardware control circuitry can be used in place of the microprocessor based circuitry described above. 

1. A medical device comprising: an input for receiving an input signal for controlling the medical device; an end effector; an ultrasonic transducer coupled to the end effector; a drive circuit coupled to the input and operable to generate a periodic drive signal and to provide the drive signal to the ultrasonic transducer; wherein the ultrasonic transducer has a resonant characteristic and wherein the drive circuit comprises a resonant circuit that is matched to the resonant characteristic of the ultrasonic transducer.
 2. A device according to claim 1, comprising a first controller for varying the period of the drive signal in dependence upon a sensed load voltage and a sensed load current applied to the transducer.
 3. A device according to claim 2, wherein the first controller is arranged to vary the period of the drive signal in dependence upon a phase difference between the sensed load voltage and the sensed load current.
 4. A device according to claim 3, wherein the first controller is arranged to vary the period of the drive signal so that the phase difference between the sensed load voltage and the sensed load current corresponds to a phase demand.
 5. A device according to claim 4, comprising a second controller that is arranged to vary the phase demand in order to vary the power, voltage or current applied to the ultrasonic transducer.
 6. A device according to claim 5, wherein the second controller is arranged to vary the phase demand in dependence upon the sensed load voltage and/or the sensed load current or on a measured load impedance.
 7. A device according to any of claims 1 to 6, comprising switching circuitry that generates a periodic drive signal from a DC supply and wherein the resonant circuit is configured to receive the periodic drive signal.
 8. A device according to any of claims 1 to 7, comprising control circuitry that is arranged to generate a digital drive signal comprising periodic drive pulses of opposite polarity and wherein the relative timing of the pulses is selected to minimise third harmonic content of the drive signal.
 9. A device according to any of claims 1 to 8, further comprising circuitry for reducing harmonic components of the drive signal that are applied to the transducer.
 10. A device according to claim 9, wherein the circuitry comprises one or more of: i) a snubber circuit connected in parallel with the transducer for providing a low impedance path for harmonic signals of the drive signal; and ii) an active feedback circuit that is arranged to generate harmonic compensation signals to cancel harmonic signals of the drive signal.
 11. A device according to any of claims 1 to 10 comprising a battery compartment for holding one or more batteries for providing power to said drive circuit for generating said drive signal.
 12. A device according to any of claims 1 to 11, comprising a control circuit operable to receive a measurement of the drive signal and operable to vary the period of the drive signal to control the power, voltage and/or current delivered to the ultrasonic transducer.
 13. A device according to claim 12, wherein the measurement is obtained from a sampling circuit that samples a sensed voltage or current signal at a sampling frequency that varies with the period and phase of the drive signal.
 14. A device according to any of claim 12 or 13, wherein the control circuit is configured to vary the period of the drive signal so that the frequency of the drive signal varies around the resonant frequency of the ultrasonic transducer, preferably within 0.1% to 1% of the resonant frequency of the ultrasonic transducer.
 15. A device according to claim 14, wherein the resonant characteristic of the ultrasonic transducer varies with a load coupled to the ultrasonic transducer during use and wherein the control circuit is configured to vary the period of the drive signal to track changes in the resonant characteristic of the ultrasonic transducer.
 16. A medical device comprising: an input for receiving an input for controlling the operation of he device; an end effector; an ultrasonic transducer coupled to the end effector; a drive circuit operable to generate a periodic drive signal and to provide the drive signal to the ultrasonic transducer; and a control circuit operable to vary the period of the drive signal to control at least one of the power, the voltage and the current provided to the ultrasonic transducer.
 17. A device according to claim 16, wherein the control circuit comprises a first controller for varying the period of the drive signal in dependence upon a sensed load voltage and a sensed load current applied to the ultrasonic transducer.
 18. A device according to claim 17, wherein the first controller is arranged to vary the period of the drive signal in dependence upon a phase difference between the sensed load voltage and the sensed load current.
 19. A device according to claim 18, wherein the first controller is arranged to vary the period of the drive signal so that the phase difference between the sensed load voltage and the sensed load current corresponds to a phase demand.
 20. A device according to claim 19, wherein the control circuit comprises a second controller that is arranged to vary the phase demand in order to vary the power, voltage or current applied to the ultrasonic transducer.
 21. A device according to claim 20, wherein the second controller is arranged to vary the phase demand in dependence upon the sensed load voltage and/or the sensed load current or on a measured load impedance.
 22. A method of operating a medical device comprising generating a periodic drive signal and applying the drive signal to an ultrasonic transducer that is coupled to an end effector of the medical device and controlling the period of the drive signal to control at least one of the power, current, and voltage applied to the ultrasonic transducer.
 23. A method of cauterising or cutting a vessel or tissue, the method comprising: gripping the vessel or tissue with an end effector of a medical device; using a drive circuit to apply a periodic drive signal to an ultrasonic transducer coupled to the end effector; and controlling the period of the drive signal to control at least one of the power, current, and voltage applied to the tissue to perform the cauterising or the cutting.
 24. A method according to claim 23, which uses the medical device of any of claims 1 to
 21. 25. A method according to claim 23 or 24, wherein said controlling step varies the period of the drive signal to control the power of the ultrasonic signal applied to the tissue or vessel, and further comprising obtaining a measurement of the impedance of a load on the drive circuit caused by the transducer and the tissue or vessel and varying a desired power to be applied to the tissue or vessel in dependence upon the obtained impedance measurement.
 26. Electronic apparatus for use in a medical device having an ultrasonic transducer, the electronic apparatus comprising: a drive circuit operable to generate a periodic drive signal for supply to the ultrasonic transducer; sensing circuitry for sensing a load voltage and/or a load current supplied to the ultrasonic transducer; a control circuit operable to vary the period of the drive signal to control at least one of the power, the voltage and the current supplied to the ultrasonic transducer in dependence upon the load voltage and/or load current sensed by the sensing circuitry.
 27. An apparatus according to claim 26, wherein the control circuit is arranged to vary the period of the drive signal in dependence upon a phase difference between the sensed load voltage and the sensed load current.
 28. An apparatus according to claim 27, wherein the control circuit is arranged to vary the period of the drive signal no that the phase difference between the sensed load voltage and the sensed load current corresponds to a phase demand.
 29. An apparatus according to claim 28, comprising a second control circuit that is arranged to vary the phase demand in order to vary the power, voltage or current applied to the ultrasonic transducer.
 30. An apparatus according to claim 29, wherein the second control circuit is arranged to vary the phase demand in dependence upon the sensed load voltage and/or the sensed load current.
 31. An apparatus according to any of claims 26 to 30, wherein the ultrasonic transducer has a resonant characteristic and the drive circuit comprises a resonant circuit that is matched with the resonant characteristic of the ultrasonic transducer; and further comprising switching circuitry that generates a periodic drive signal from a DC supply and wherein the resonant circuit is configured to receive the periodic drive signal.
 32. A medical device or control circuitry substantially as described hereinabove with reference to or as shown in the accompanying drawings.
 33. A method of operating a medical device substantially as described hereinabove with reference to or as shown in the accompanying drawings. 